The present invention relates generally to ferromagnetic object detectors and more particularly to a passive coil inductor system for detecting ferromagnetic objects which are passed through the system or to which the system is passed in proximity.
Metal detectors are used for security purposes in a number of locations, such as airports, federal buildings, banks, schools and other high-security installations. Currently, there are two types of metal detectors in use in such installations. The first type includes a transmitting coil located on one side of the detector and a receiving coil located on the opposite side of the detector. Typically, the magnetic field is generated on one side of the detector by the transmitting coil, and the generated field is received on the other side of the detector by the receiving coil. As long as the magnetic field received by the receiving coil is within the predetermined parameters programmed into the detector, an alarm is not actuated. However, the passage or presence of a ferromagnetic object through or in the magnetic field causes a disturbance in the field received by the receiving coil. If this disturbance causes the magnetic field to fall outside of the predetermined parameters, the alarm associated with the detector is actuated.
Another type of ferromagnetic metal detector is disclosed in U.S. Pat. No. 3,971,983 to Jaquet. This detector employs a number of gradiometers positioned on both sides of a walk-through portal. While this device does not actively generate a magnetic field within the portal, the gradiometers monitor the magnetic field generated by the earth. Any disturbances in the earth""s magnetic field, such as may be caused by the presence of a ferromagnetic object within the portal, are detected by the gradiometers, resulting in the activation of an alarm.
While these types of detector systems can be very accurate, the operation of and hardware associated with the system are very complex and these systems are very expensive to manufacture and operate. Furthermore, these systems can be very sensitive to changes in the magnetic field which occur outside of the detectors and which are not caused by the contraband which the detectors are designed to detect, thereby resulting in undesired false alarms.
What is needed therefore is a ferromagnetic metal detecting system which is inexpensive to manufacture and operate and which results in less false detections and consequently, less false alarms.
The present invention includes a ferromagnetic metal detector which is simple and inexpensive to manufacture and operate and which is also less prone to interference from ferromagnetic materials which are not passed through the detector. The simple design of the invention also allows the detector to be easily transportable, thereby allowing the detector to be moved between different locations that require monitoring. The detector of the present invention includes a pair of vertically aligned inductive coils located to define a passageway therebetween. The inductive coils are electrically coupled together and to a control device. Movement of a ferromagnetic object in proximity of the inductive coils induces a voltage within the coils, which voltage may then used to activate an alarm.
According to one embodiment of the invention, a device for detecting the presence of a ferromagnetic object is disclosed, comprising at least one coil formed from a multiple winding of a wire and a controller comprising an alarm. The coil is constructed and arranged such that, upon the relative motion of the ferromagnetic object proximate the at least one coil, a voltage induced in the coil is transmitted to the controller, and controller is constructed and arranged to determine whether the induced voltage falls within a predetermine range and, if so, to trigger the alarm.
Other features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.